With the advance of laser processing machines, in recent years, magneto-optical devices utilizing interactions between light and magnetism have come to draw attention. One of these devices is an isolator, which is for a function to inhibit the phenomenon in which if light oscillated from a laser source is reflected by an optical system in its path and returns to the light source, then it disturbs the light oscillated from the laser source, to cause an unstable oscillation state. Therefore, the optical isolator is used in the state of being disposed between a laser source and an optical component, utilizing this function.
An optical isolator includes three component parts, namely, a Faraday rotator, a polarizer disposed on the light incoming side of the Faraday rotator, and an analyzer arranged on the light outgoing side of the Faraday rotator. The optical isolator utilizes the nature, so-called Faraday effect, that when light enters the Faraday rotator under the condition where a magnetic field is applied to the Faraday rotator in a direction parallel to the traveling direction of the light, the plane of polarization is rotated in the Faraday rotator. Specifically, that component of the incident light which has the same plane of polarization as that of the polarizer is transmitted through the polarizer and enters the Faraday rotator. This light is rotated by +45 degrees relative to the light traveling direction in the Faraday rotator, before going out therefrom.
On the other hand, when the return light entering the Faraday rotator in an opposite direction to the incident direction first passes through the analyzer, only that component of the light which has the same plane of polarization as that of the analyzer is transmitted through the analyzer and enters the Faraday rotator. Next, in the Faraday rotator, the plane of polarization of the return light is further rotated by +45 degrees in addition to the initial +45 degrees. Since the plane of polarization of the return light is at a right angle of +90 degrees with respect to the polarizer, therefore, the return light cannot pass through the polarizer.
The Faraday angle θ is represented by the following formula (A).θ=V×H×L  (A)
In the formula (A), V is a Verdet constant which is determined by the material of the Faraday rotator, H is magnetic flux density, and L is the length of the Faraday rotator. For use as an optical isolator, L is determined such that θ=45 degrees.
It is important that the material to be used for the Faraday rotator of the optical isolator as above-mentioned have a large Faraday effect and a high transmittance at the wavelength at which it is used.
Further, if a polarized component different from the incident light is generated in the outgoing light, this different polarized component is transmitted through the polarizer, resulting in insufficient blockage of the return light.
For evaluation of the generation of such a different polarized component, polarized light of 0 to 90 degrees is made to enter a material used as the Faraday rotator, the outgoing light is transmitted through the polarizer into a photodetector, and the light intensity is measured by the photodetector. From the maximum intensity (Imax) and minimum intensity (Imin), an extinction ratio (S) is calculated according to the following formula.S=−10 log(Imin/Imax)[unit:dB]
Higher values of extinction ratio are important, and, in general, an extinction ratio of at least 30 dB is required.
Recently, as such material, JP-A 2010-285299 (Patent Document 1) discloses a single crystal oxide and transparent oxide ceramics of (TbxRe1-x)2O3 (wherein 0.4≤x≤1.0), that is the material having a high Verdet constant.
Also, JP 4033451 (Patent Document 2) discloses that rare earth oxides represented by the general formula:R2O3 (wherein R is a rare earth element) are free of birefringence since their crystal structures are cubic, and, accordingly, sintered bodies having a high degree of transparency can be produced if pores and impurity segregates are completely removed.
Further, JP-A H05-330913 (Patent Document 3) discloses that addition of a sintering aid is effective for removing pores. Also, JP 2638669 (Patent Document 4) discloses a method for removing pores by a hot isostatic press followed by re-sintering. A production method involves adding one or more of the sintering aids disclosed in Patent Document 3 or the like, mixing, compacting, calcining, sintering in vacuum, and a HIP (Hot Isostatic Press) treatment.
In Patent Document 1, the transparent oxide ceramics of (TbxRe1-x)2O3 (wherein 0.4≤x≤1.0) basically have a cubic crystal structure. However, these transparent oxide ceramics sometimes exhibit faint birefringence because a sintering aid incorporated therein can react with the main component to form a phase different from the cubic crystal that precipitates within crystal grains or at grain boundaries. This may result in a lowering in extinction ratio.
In addition, since the precipitates are of a minute size of up to 1 μm, laser light applied to the ceramic material is scattered there. Due to the scattering, the characteristics of insertion loss may be worse.
Besides, when the ceramic materials are sintered, the composition of the main component (TbxRe1-x)2O3 and the concentration of the sintering aid vary between the inside and the outer periphery of the ceramic material due to segregation, resulting in variations of extinction ratio and insertion loss within the ceramic surface.
Further, JP-A 2012-206935 (Patent Document 5) states that an optical ceramic material containing rare earth oxides as main components and having a minimal compositional variation and optical uniformity, with an insertion loss of up to 1.0 dB, specifically, a ceramic which includes a compound of terbium oxide (chemical formula: Tb2O3) with at least one oxide selected from among yttrium oxide, scandium oxide and lanthanide rare earth oxide having little absorption at a wavelength of 1,064 nm and which contains terbium oxide in a molar ratio of at least 40% is successfully obtained by:
(1) using a starting material having excellent sinterability and a specific particle size distribution;
(2) using a sintering aid having excellent sinterability and capable of maintaining the ceramic crystal structure cubic;
(3) performing sintering in vacuum or an oxygen-free non-oxidizing atmosphere at an optimum temperature and a HIP treatment; and
(4) subjecting the sintered body obtained in this manner to a heat treatment in an oxygen-free atmosphere,
for reducing heterophase precipitates which might cause scattering or pores.
In recent years, the outputs of laser processing machines have been being further raised beyond 10 W, and it has become possible to obtain laser processing machines with a high output ranging from 20 W to several hundreds of watts. In this case, since high-power laser light passes through a Faraday optical material of an isolator which is one of component parts of the laser processing machine, the lowering in the laser output upon the passage through the Faraday optical material cannot be ignored, even where the insertion loss is approximately 1.0 dB. Further, if the radiated light is absorbed by the Faraday optical material, the part where the light absorption is occurring and the surrounding part undergo variations in density and refractive index. Specifically, when high-intensity light such as laser light is applied to a Faraday optical material, a temperature distribution is generated in the Faraday optical material between a central portion where heat accumulation is liable to occur and the outside portion where heat is radiated, and the refractive index of the part where the laser light passes is changed more greatly than the refractive index of the surrounding part, resulting in a lens effect. As a result, the laser light which intrinsically should go out in parallel would be focused. This phenomenon, called thermal lens, has been an unstable factor with respect to stable oscillation of laser light.
In addition, if an optical material has a defect that causes light absorption, the light absorbed in the defect is converted into heat, making the thermal lens problem graver.
In view of this, for reducing the thermal lens, it is necessary to further lessen the defects which cause light absorption. Examples of such defects include heterophase precipitates, and pores, oxygen defects of ionic defects which are present at grain boundaries or within crystal grains.